Multi-core fiber coupler based on single-clad fiber and its tapered ratio design method

Abstract

The present invention relates to the technical field of optical fibers, and proposes a multi-core optical fiber coupler based on a single-clad optical fiber and a tapered ratio design method thereof. The single-clad optical fiber (1) comprises a core (13), a cladding (12) and a coating (11) arranged sequentially from the inside to the outside; the multi-core optical fiber coupler based on the single-clad optical fiber comprises a single-clad optical fiber (1) and a sleeve (2); the sleeve (2) is embedded with a plurality of single-clad optical fibers (1), and the total number of the single-clad optical fibers (1) is equal to the total number of cores in a target multi-core optical fiber (3); one end of the sleeve (2) is tapered, and the other end is not tapered; and the mode field diameter of the single-clad optical fiber (1) in the end of the sleeve (2) that is not tapered matches the mode field diameter of a single core of the target multi-core optical fiber (3); the single-clad optical fiber and the multi-core optical fiber coupler have the advantages of low preparation difficulty, low cost, and the ability to achieve low-loss connection with a multi-core optical fiber and a standard single-mode optical fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and more particularly to a multi-core optical fiber coupler based on single-clad optical fibers and a method for designing a tapered ratio thereof. Background Art

[0002] The rapid spread and development of internet technology has fundamentally changed the way people live and work, significantly promoting economic and social transformation and development. At the same time, this has also led to a growing demand for data traffic. The current backbone network, based on single-mode fiber, is approaching its maximum transmission capacity limit and cannot meet the upcoming demand for the interconnection of everything.

[0003] Space-division multiplexing technology based on multi-core fiber, which leverages the presence of multiple cores within a single cladding structure to provide a new dimension of spatial multiplexing and exponentially increase the capacity of communication systems, has become a research hotspot. In the application of multi-core fiber, in order to be compatible with existing communication systems based on standard single-mode fiber, multi-core fiber couplers that can provide low-loss connections between standard single-mode fiber and multi-core fiber are essential. Therefore, the design and preparation of multi-core fiber coupling devices has become one of the key technologies for promoting multi-core fiber.

[0004] At present, the preparation methods of all-fiber multi-core fiber couplers mainly include micro-hole processing, corroded fiber bundle taper method and multi-clad fiber taper method.

[0005] The micro-hole processing method inserts a standard single-mode fiber and a multi-core fiber, which have undergone etching, into a cylindrical sleeve that has been mechanically drilled or laser-drilled. Alignment is achieved using an alignment platform, and then the fiber is fixed with glue. However, due to issues with etching and drilling precision, the coupler has high loss.

[0006] The corroded fiber bundle taper method inserts corroded standard single-mode optical fibers into a ferrule, tapers them, and then cuts and polishes them before connecting them to a multi-core optical fiber to create a coupler. Corroded optical fibers are prone to dimensional deviation and increased brittleness, resulting in high insertion loss and increased fiber breakage.

[0007] The multi-clad fiber taper method uses optical fibers with multiple cladding layers to create a tapered connection. By optimizing the refractive index and dimensions of the different cladding layers, the loss of multi-core fiber couplers can be effectively reduced. However, due to the multi-cladding structure of the optical fiber, interference effects are introduced during the taper process, limiting the spectral range of the transmissive light and causing transmission loss to fluctuate significantly with wavelength.

[0008] In summary, the current preparation methods of various multi-core fiber couplers have problems in terms of corrosion accuracy, fiber geometric alignment accuracy, processing difficulty, cost and loss. Summary of the Invention

[0009] The present invention aims to overcome the defects of the above-mentioned prior art multi-core fiber couplers, such as the difficulty in preparation, high preparation cost and large connection loss, and provide a multi-core fiber coupler based on single-clad optical fiber with low preparation difficulty, low preparation cost and low connection loss.

[0010] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0011] A single-clad optical fiber comprises a core, a cladding and a coating layer which are sequentially arranged from the inside to the outside.

[0012] The present invention also proposes a multi-core fiber coupler based on a single-clad fiber, comprising a single-clad fiber and a sleeve;

[0013] The sleeve is nested with a plurality of single-clad optical fibers, and the total number of the single-clad optical fibers is equal to the total number of cores in the target multi-core optical fiber;

[0014] One end of the sleeve is tapered, and the other end is not tapered; the tapered end of the sleeve is used to connect to the target multi-core optical fiber; each single-clad optical fiber in the untapered end of the sleeve is used to connect to a standard single-mode optical fiber, and the mode field diameter of the single-clad optical fiber in the untapered end of the sleeve matches the mode field diameter of a single core of the target multi-core optical fiber.

[0015] The present invention also proposes a method for designing a tapering ratio based on a multi-core optical fiber coupler, which uses the above-mentioned multi-core optical fiber coupler based on single-clad optical fiber, including the following steps:

[0016] Adjusting the parameters of the single-clad optical fiber according to the mode field diameter A of a single core in the target multi-core optical fiber so that the difference between the mode field diameter B and the mode field diameter A of the single-clad optical fiber in the end of the sleeve that has not been tapered is no greater than a preset threshold;

[0017] Let the tapering ratio of the multi-core fiber coupler be The initial value is 1, and the taper ratio is Calculate the mode field diameter C of the single-clad optical fiber in the tapered end of the ferrule when setting it as a decreasing variable;

[0018] When the cone ratio When the value is not 1 and the mode field diameter C is equal to the mode field diameter B, the calculation is stopped and the taper ratio at this time is set to The size of , get the taper ratio The optimal value range of .

[0019] The present invention also proposes a design method based on a multi-core optical fiber coupler, comprising the following steps:

[0020] According to the core spacing of the target multi-core fiber , calculate the cladding diameter of the single-clad optical fiber in the end of the sleeve that has not been tapered , the cladding diameter The calculation expressions include:

[0021]

[0022] Where, represents the tapering ratio of the multi-core optical fiber coupler;

[0023] Determine the cladding diameter After the size of the target multi-core optical fiber is determined, the diameter of the circumscribed circle of the single-clad optical fiber under the tight arrangement is The size of is set to the optimal inner diameter of the sleeve;

[0024] According to the cladding diameter of the target multi-core fiber Calculate the minimum outer diameter of the casing , the minimum outer diameter The expressions include:

[0025] .

[0026] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0027] The single-clad optical fiber includes a core, a cladding, and a coating layer arranged sequentially from the inside to the outside, and has a simple structure. Compared with a double-clad optical fiber, its preparation is less difficult and has a lower preparation cost. The total number of the single-clad optical fibers in the multi-core optical fiber coupler based on the single-clad optical fiber is equal to the total number of cores in the target multi-core optical fiber, and the mode field diameter of the single-clad optical fiber in the end of the sleeve that has not been tapered matches the mode field diameter of a single core of the target multi-core optical fiber, which can effectively reduce the connection loss between the multi-core optical fiber coupler and the target multi-core optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic structural diagram of the single-clad optical fiber proposed in Example 1;

[0029] Figure 2 This is a schematic structural diagram of the multi-core fiber coupler based on single-clad fiber proposed in Example 1;

[0030] Figure 3 This is a schematic diagram of a small-scale taper proposed in Example 1;

[0031] Figure 4This is the refractive index profile of the single-clad optical fiber proposed in Example 1;

[0032] Figure 5 This is a schematic diagram of the structure of the multi-core optical fiber proposed in Example 1;

[0033] Figure 6 Schematic diagram of the changes in mode field diameter and loss during the small-ratio tapering process of the single-clad optical fiber proposed in Example 3;

[0034] Figure 7 This is a schematic diagram of the change in coupling loss corresponding to different cladding diameter values of the single-clad optical fiber proposed in Example 3;

[0035] Among them, 1-single-clad fiber bundle, 11-single-clad fiber coating, 12-single-clad fiber cladding, 13-single-clad fiber core, 2-casing, 3-target multi-core fiber, 31-multi-core fiber coating, 32-multi-core fiber cladding, 33-multi-core fiber core. DETAILED DESCRIPTION

[0036] The accompanying drawings are for illustrative purposes only and are not to be construed as limiting the present embodiment;

[0037] In order to better illustrate this embodiment, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product size;

[0038] It is understandable to those skilled in the art that some well-known structures and descriptions thereof may be omitted in the drawings.

[0039] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0040] Example 1

[0041] This embodiment proposes a single-clad optical fiber. Figure 1 This is a schematic structural diagram of the single-clad optical fiber proposed in this embodiment.

[0042] The single-clad optical fiber 1 includes a core 13 , a cladding 12 and a coating 11 arranged sequentially from the inside to the outside.

[0043] In this embodiment, Figure 1 This is a schematic diagram of the structure of the single-clad optical fiber proposed in this embodiment; Figure 1 As shown, the preparation process of the single-clad optical fiber with one cladding layer is simple, does not require treatment such as corrosion, has good scalability, and has a high yield.

[0044] In an optional embodiment, the refractive index at the interface between the core 13 and the cladding 12 changes suddenly, and the refractive index at the interface between the cladding 12 and the coating 11 changes suddenly, the refractive index of the cladding 12 is smaller than the refractive index of the core 13, and the refractive index of the coating 11 is smaller than the refractive index of the cladding 12.

[0045] In an optional embodiment, the refractive index of the fiber core 13 is in a step-type distribution or a gradient-type distribution.

[0046] In a specific implementation process, the single-clad optical fiber includes a core, a cladding and a coating layer arranged sequentially from the inside to the outside, and has a simple structure. Compared with the double-clad optical fiber, its preparation difficulty and preparation cost are lower.

[0047] This embodiment also proposes a multi-core fiber coupler based on single-clad fiber. Figure 2 FIG. 4 is a schematic structural diagram of a multi-core fiber coupler based on a single-clad optical fiber according to an embodiment of the present invention.

[0048] The multi-core fiber coupler based on single-clad fiber proposed in this embodiment includes a single-clad fiber 1 and a sleeve 2;

[0049] The sleeve 2 is nested with a plurality of single-clad optical fibers 1, and the total number of the single-clad optical fibers 1 is equal to the total number of cores in the target multi-core optical fiber 3;

[0050] One end of the sleeve 2 is tapered, and the other end is not tapered; the end of the sleeve 2 that is tapered is used to connect to the target multi-core optical fiber 3; each single-clad optical fiber 1 in the end of the sleeve 2 that is not tapered is used to connect to a standard single-mode optical fiber, and the mode field diameter of the single-clad optical fiber 1 in the end of the sleeve 2 that is not tapered matches the mode field diameter of a single core of the target multi-core optical fiber 3.

[0051] During the specific implementation process, the total number of the single-clad optical fibers in the multi-core optical fiber coupler is equal to the total number of cores in the target multi-core optical fiber, and the mode field diameter of the single-clad optical fiber in the end of the sleeve that has not been tapered matches the mode field diameter of a single core of the target multi-core optical fiber, which can effectively reduce the connection loss between the multi-core optical fiber coupler and the target multi-core optical fiber.

[0052] In an optional embodiment, the optical fiber bundle composed of the plurality of single-clad optical fibers 1 is nested into the sleeve 2 according to the core arrangement of the target multi-core optical fiber 3;

[0053] One end of the ferrule 2 in which several single-clad optical fibers 1 are nested is tapered and the flat area of the tapered ferrule 2 is cut, and then connected to the target multi-core optical fiber 3.

[0054] In an optional embodiment, the tapering ratio of the multi-core optical fiber coupler is The expressions include:

[0055]

[0056] Where, represents the tapering ratio of the multi-core optical fiber coupler; represents the diameter of the core 13 of the single-clad optical fiber 1 in the tapered end of the sleeve 2; represents the diameter of the core 13 of the single-clad optical fiber 1 in the end of the sleeve 2 that is not subjected to the tapering process;

[0057] The tapering ratio of the multi-core optical fiber coupler The optimal value range includes , where the cone ratio is When the taper ratio is 1, it is considered that both ends of the sleeve 2 are not tapered. for When the mode field diameter of the single-clad optical fiber 1 in the end of the sleeve 2 that is not subjected to the tapering treatment is equal to the mode field diameter of the single-clad optical fiber 1 in the end of the sleeve 2 that is subjected to the tapering treatment.

[0058] In an optional embodiment, the expression for the diameter of the cladding 12 of the single-clad optical fiber 1 in the end of the sleeve 2 that is not subjected to the tapering process includes:

[0059]

[0060] Where, represents the diameter of the cladding 12 of the single-clad optical fiber 1 in the end of the sleeve 2 that is not subjected to the tapering process; represents the core spacing of the target multi-core optical fiber 3; represents the tapering ratio of the multi-core fiber coupler.

[0061] In an optional embodiment, the expression for the minimum outer diameter of the sleeve 2 includes:

[0062]

[0063] Where, Indicates the minimum outer diameter of the casing 2; represents the cladding diameter of the target multi-core optical fiber 3; represents the tapering ratio of the multi-core fiber coupler.

[0064] Optionally, the sleeve 2 is a hollow sleeve.

[0065] Optionally, the connection method between the single-clad optical fiber 1 and the target multi-core optical fiber 3 includes welding or bonding with glue; optionally, when the welding method is used, the welding is achieved by a welding machine, and the heat source of the welding machine is arc discharge, carbon dioxide laser or graphite.

[0066] Optionally, the diameter of the cladding 12 is less than 100 microns.

[0067] Optionally, the diameter of the fiber core 13 is 4-15 microns, and the relative refractive index difference with the cladding 12 is 0.003-0.007.

[0068] As an example, Figure 3 This is a schematic diagram of a small-scale taper proposed in this embodiment; Figure 4 This is the refractive index distribution diagram of the single-clad optical fiber proposed in this embodiment, where: Figure 4 The refractive index of the core 13 is in a step-type distribution; Figure 5 This is a schematic diagram of the structure of the multi-core optical fiber proposed in this embodiment. The structure of the multi-core optical fiber is as follows Figure 5 As shown, 31 represents the coating layer of the multi-core optical fiber, 32 represents the cladding layer of the multi-core optical fiber, and 33 represents the core of the multi-core optical fiber.

[0069] The tapered ratio The optimal value range of is considered as a small-scale taper, such as Figure 3 As shown, the characteristic of the gradual change of the mode field diameter of the optical fiber under small-ratio tapering is utilized, that is, the mode field diameter of the single-clad optical fiber changes very little under the condition of small-ratio tapering, which makes the mode field diameter of the optical fiber after tapering close to the mode field diameter of the single core of the multi-core optical fiber, so the loss is very small. It should be noted that under the condition of small-ratio tapering, the light of the single-clad optical fiber is still confined to the original core for transmission; and the single-clad optical fiber is a simple single-core / single-cladding structure (it only has a core, a cladding and a mechanical cladding); the optical fiber used in the prior art is a double-clad optical fiber, which has a core, a first outer cladding, a depressed outer cladding and a mechanical cladding, which greatly simplifies the difficulty and cost of preparing the optical fiber, and in the process of tapering, the double-clad optical fiber actually corresponds to Figure 3 The fiber bundle undergoes a large-scale tapering (corresponding to the mode field diffusion region). Specifically, as the fiber bundle tapers, the core size, first outer cladding, and depressed cladding dimensions decrease simultaneously, until the original core structure can no longer support light transmission, and light leaks from the core into the first outer cladding. At this point, due to the presence of the depressed outer cladding, the first outer cladding / depressed outer cladding form an optical waveguide transmission structure. Therefore, in actual design, it is often necessary to adjust the refractive index difference between the core / first outer cladding and the first outer cladding / depressed outer cladding to bring the fiber's mode field diameter close to that of a single core in the target multi-core fiber.

[0070] In the solution proposed in this application, Figure 4 As shown in the figure, in the case of small-ratio tapering, the light is bound in the fiber core. It is sufficient for the initial mode field diameter of the single-clad fiber to be close to the mode field diameter of a single core of the multi-core fiber. By utilizing the characteristic of the gradual change of the mode field of small-ratio tapering, a low-loss connection can be achieved after the fiber bundle is tapered at a small ratio.

[0071] Preferably, the refractive index selection parameters of the single-clad optical fiber are the same as the refractive index of the multi-core optical fiber core; in this way, there is no refractive index difference between the two at the connection interface of the optical fiber bundle / multi-core optical fiber, and no back reflection will be caused, which is beneficial to reduce signal interference in high-speed communication systems.

[0072] However, in the existing multi-core fiber coupler corresponding to double-clad optical fiber, there is a difference in the refractive index of the tapered fiber bundle / multi-core optical fiber, which will cause power backreflection, thus hindering the transmission of the communication system.

[0073] To sum up, the present application utilizes the characteristic of gradual change of mode field when optical fiber is tapered at a small ratio, and its core size and core / cladding refractive index can be consistent with those of multi-core optical fiber. Therefore, when connected to multi-core optical fiber, it has lower connection loss and large return loss; and the present application utilizes the characteristic of gradual change of mode field when optical fiber is tapered at a small ratio, which can limit the coverage range of the field and avoid field leakage to the outside. Therefore, after connecting to multi-core optical fiber, it has smaller inter-core crosstalk.

[0074] Example 2

[0075] This embodiment proposes a tapering ratio design method based on a multi-core fiber coupler, and applies the multi-core fiber coupler based on a single-clad fiber described in Example 1.

[0076] The method for designing a tapering ratio based on a multi-core optical fiber coupler comprises the following steps:

[0077] S1: adjusting the parameters of the single-clad optical fiber 1 according to the mode field diameter A of a single core in the target multi-core optical fiber 3, so that the difference between the mode field diameter B and the mode field diameter A of the single-clad optical fiber 1 in the end of the sleeve 2 that has not been tapered is less than a preset threshold;

[0078] S2: The tapering ratio of the multi-core fiber coupler The initial value is 1, and the taper ratio is When the variable is set as a decreasing variable, the mode field diameter C of the single-clad optical fiber 1 in the tapered end of the sleeve 2 is calculated;

[0079] S3: When the pull cone ratio When the value is not 1 and the mode field diameter C is equal to the mode field diameter B, the calculation is stopped and the taper ratio at this time is set to The size of , get the taper ratio The optimal value range of .

[0080] As an exemplary illustration, based on the mode field diameter A of a single fiber core in the target multi-core optical fiber 3, the diameter d1 of the core 13 and the relative refractive index difference parameters of the single-clad optical fiber 1 are designed so that the difference between the mode field diameter B and the mode field diameter A of the single-clad optical fiber 1 is less than a preset threshold; wherein, the smaller the difference between the mode field diameter B and the mode field diameter A of the single-clad optical fiber 1, the smaller the coupling loss of the multi-core optical fiber coupler.

[0081] This embodiment also proposes a design method based on a multi-core optical fiber coupler, and applies the multi-core optical fiber coupler based on a single-clad optical fiber described in Example 1.

[0082] The design method based on the multi-core optical fiber coupler comprises the following steps:

[0083] S1: According to the core spacing of the target multi-core optical fiber 3 , calculate the diameter of the cladding 12 of the single-clad optical fiber 1 in the end of the sleeve 2 that is not tapered , the cladding 12 diameter The calculation expressions include:

[0084]

[0085] Where, represents the tapering ratio of the multi-core optical fiber coupler;

[0086] S2: Determine the diameter of the cladding 12 After the size of the target multi-core optical fiber 3 is arranged, the diameter of the circumscribed circle of the single-clad optical fiber 1 is tightened and arranged. The size is set to the optimal inner diameter of the sleeve 2;

[0087] S3: According to the cladding diameter of the target multi-core optical fiber 3 Calculate the minimum outer diameter of the sleeve 2 , the minimum outer diameter The expressions include:

[0088] .

[0089] In this optional embodiment, simulation software or numerical calculation methods are used to calculate the change in mode field diameter of a single-clad optical fiber (small clad diameter optical fiber) 1 under tapering, and the value range of the tapering ratio is recorded [R, 1], where 1 indicates that the single-clad optical fiber 1 is not tapered, and the mode field diameter at this time is B*1=B. As the tapering progresses, the cladding and core diameters of the single-clad optical fiber 1 decrease in the same proportion, and the mode field diameter B will experience a process of first decreasing and then increasing; the tapering ratio when the mode field diameter increases to B is recorded as R. Where R is [0, 1]; utilizing the characteristic that the mode field diameter of the optical fiber changes slowly within the small-ratio tapering range [1, R], when the tapering ratio r of the device is included in [R, 1], a low-loss connection between the single-clad optical fiber 1 and the multi-core optical fiber 3 after tapering can be achieved;

[0090] According to the core spacing Λ of the multi-core optical fiber 3, the diameter d2 of the cladding 12 of the single-clad optical fiber 1 can be obtained by the formula d2=Λ / r; based on the above-mentioned parameter of the diameter d2 of the cladding 12 of the single-clad optical fiber 1, according to the core arrangement scheme of the multi-core optical fiber 3, the circumscribed circle diameter D1 of the single-clad optical fiber 1 under the tight arrangement is determined as the optimal inner diameter of the sleeve 2; according to the cladding diameter d3 of the multi-core optical fiber 3, the minimum outer diameter D2 of the sleeve 2 can be obtained by the formula D2=d3 / r.

[0091] This embodiment also provides a method for preparing a multi-core optical fiber coupler, which uses the multi-core optical fiber coupler based on single-clad optical fiber described in Example 1, including the following steps:

[0092] S1: After an optical fiber bundle including several single-clad optical fibers 1 is nested into a sleeve 2 according to the core arrangement of a target multi-core optical fiber 3, the sleeve 2 is tapered;

[0093] S2: After cutting the flat area of the tapered sleeve 2, the sleeve 2 is connected to the target multi-core optical fiber 3;

[0094] S3: Connect each single-clad optical fiber 1 in the end of the sleeve 2 that has not been tapered to a standard single-mode optical fiber.

[0095] This embodiment proposes a computer device including a memory and a processor, wherein the memory stores computer-readable instructions. When the computer-readable instructions are executed by the processor, the processor executes the steps of the tapering ratio design method, design method or preparation method of the multi-core optical fiber coupler described in Example 1.

[0096] Example 3

[0097] This embodiment provides specific implementation examples of the tapering ratio design method, design method, and preparation method based on the multi-core optical fiber coupler.

[0098] Figure 6Schematic diagram of the change of the mode field diameter of the single-clad optical fiber proposed in this embodiment with the taper ratio; Figure 7 This is a schematic diagram of the change in coupling loss corresponding to different cladding diameter values of the single-clad optical fiber proposed in this embodiment;

[0099] During the specific implementation process, a corresponding number of single-clad optical fibers 1 are prepared according to the number of cores of the multi-core optical fiber 3, and are inserted into a tapered sleeve 2 for small-scale tapering. After tapering, the flat area of the device is cut and connected to the multi-core optical fiber 3; this overcomes the technical problems of connecting the multi-core optical fiber and the single-mode optical fiber, and utilizes the characteristic of the slow change of the mode field under the condition of small-scale tapering to effectively reduce the connection loss, return loss and inter-core crosstalk of the multi-core optical fiber coupler.

[0100] The tapered sleeve 2 is a glass tube, and the multi-core fiber 3 is a four-core fiber. The parameters of the four-core fiber are a cladding diameter d3 of 125 microns, a core spacing Λ of 42 microns, and a mode field diameter A of 9.5 microns at 1550 nm. To demonstrate the universality of the proposed method, the parameters of the single-clad fiber 1 are selected. A single-mode fiber meeting the G652.D fiber standard is selected, with a core diameter 13 of 9.5 microns and a relative refractive index difference of 0.0042. The calculated mode field diameter of the fiber is 10.2 microns.

[0101] Next, the simulation calculates the change of the mode field diameter of the single-clad optical fiber with the taper ratio, such as Figure 6 The dashed line shows the change in mode field diameter during the tapering process for a single-clad fiber. The mode field diameter first decreases and then increases as the tapering ratio decreases. When the tapering ratio reaches 0.7, the mode field diameter returns to its untapered value. During this process, the mode field diameter changes by only 3.03%. The coupling loss compared to a multicore fiber with a single core mode field diameter of 9.5 μm is less than 0.03 dB.

[0102] When the tapering ratio r is in the range of [0.7, 1], any tapering ratio can achieve low-loss connection between the small-clad fiber and the core of the multi-core fiber after tapering. Figure 7 The corresponding changes in coupling loss for different cladding diameters of single-clad optical fibers are given. We select two special points for illustration. When the P1 taper ratio is 0.84, the difference in mode field diameters between the two is minimal, and the coupling loss is less than 0.01dB. According to the formula d2=Λ / r, the cladding diameter of the small-clad optical fiber should be 50 microns. When we select point P2, the taper ratio is 0.7, the difference in mode field diameters between the two is maximum, and the coupling loss also reaches a maximum, but is still less than 0.03dB. According to the formula d2=Λ / r, the cladding diameter of the small-clad optical fiber should be 60 microns.

[0103] When the cladding diameter of a single-clad fiber is set to 60 μm, the corresponding circumscribed diameter of the four-core fiber arrangement is 145 μm, meaning the optimal inner diameter of sleeve 2 is 145 μm. Since the cladding diameter d3 of multi-core fiber 3 is 125 μm, the minimum outer diameter D2 of sleeve 2 is 178 μm, as calculated by the formula D2 = d3 / r.

[0104] After determining the parameters of all materials, a fiber bundle containing four single-clad optical fibers is nested into sleeve 2 according to the core arrangement of a four-core optical fiber and tapered at a ratio of 0.7. After cutting the flat area of the tapered sleeve, it is connected to the multi-core optical fiber. The fiber bundles of the untapered single-clad optical fibers are connected to standard single-mode optical fibers.

[0105] The same or similar reference numerals correspond to the same or similar components;

[0106] The terms used in the drawings to describe positional relationships are for illustrative purposes only and should not be construed as limiting the present embodiment.

[0107] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A multi-core fiber coupler based on single-clad fiber, characterized in that: It includes a single-clad optical fiber (1) and a sleeve (2); The sleeve (2) is nested with a plurality of single-clad optical fibers (1), and the total number of the single-clad optical fibers (1) is equal to the total number of cores in the target multi-core optical fiber (3); One end of the sleeve (2) is subjected to a tapered process, and the other end is not subjected to a tapered process; the end of the sleeve (2) subjected to the tapered process is used to connect to a target multi-core optical fiber (3); each single-clad optical fiber (1) in the end of the sleeve (2) not subjected to the tapered process is used to connect to a standard single-mode optical fiber, and the mode field diameter of the single-clad optical fiber (1) in the end of the sleeve (2) not subjected to the tapered process matches the mode field diameter of a single core of the target multi-core optical fiber (3); The optical fiber bundle composed of the plurality of single-clad optical fibers (1) is nested in the sleeve (2) according to the core arrangement of the target multi-core optical fiber (3); The tapering ratio of the multi-core optical fiber coupler The expressions include: Where, represents the tapering ratio of the multi-core optical fiber coupler; represents the diameter of the core (13) of the single-clad optical fiber (1) in the tapered end of the sleeve (2); represents the diameter of the core (13) of the single-clad optical fiber (1) in the end of the sleeve (2) that is not subjected to the tapering process; The tapering ratio of the multi-core optical fiber coupler The optimal value range includes ,in, , when the cone ratio When the taper ratio is 1, both ends of the casing (2) are not tapered. for When the mode field diameter of the single-clad optical fiber (1) in the end of the sleeve (2) that has not been subjected to the tapering treatment is equal to the mode field diameter of the single-clad optical fiber (1) in the end of the sleeve (2) that has been subjected to the tapering treatment; The expression for the diameter of the cladding (12) of the single-clad optical fiber (1) in the end of the sleeve (2) that is not subjected to the tapering process includes: Where, represents the diameter of the cladding (12) of the single-clad optical fiber (1) in the end of the sleeve (2) that has not been tapered; represents the core spacing of the target multi-core optical fiber (3); represents the tapering ratio of the multi-core optical fiber coupler; The expression for the minimum outer diameter of the casing (2) includes: Where, Indicates the minimum outer diameter of the casing (2); represents the cladding diameter of the target multi-core optical fiber (3); represents the tapering ratio of the multi-core optical fiber coupler; The single-clad optical fiber (1) comprises a core (13), a cladding (12) and a coating layer (11) which are arranged in sequence from the inside to the outside.

2. The multi-core fiber coupler based on single-clad fiber according to claim 1, characterized in that: The refractive index of the interface between the core (13) and the cladding (12) changes suddenly, and the refractive index of the interface between the cladding (12) and the coating (11) changes suddenly, the refractive index of the cladding (12) is less than the refractive index of the core (13), and the refractive index of the coating (11) is less than the refractive index of the cladding (12).

3. The multi-core fiber coupler based on single-clad fiber according to claim 1, characterized in that: The refractive index of the fiber core (13) is in a step-type distribution or a gradient-type distribution.

4. The multi-core fiber coupler based on single-clad fiber according to claim 1, characterized in that: One end of a sleeve (2) in which a plurality of single-clad optical fibers (1) are nested is subjected to a tapered process and a flat area of the tapered sleeve (2) is cut, and then connected to a target multi-core optical fiber (3).

5. A design method based on a multi-core fiber coupler, applied to the multi-core fiber coupler based on a single-clad fiber according to claim 1, characterized in that: The following steps are involved: According to the core spacing of the target multi-core optical fiber (3) , calculate the cladding (12) diameter of the single-clad optical fiber (1) in the end of the sleeve (2) that is not tapered , the cladding (12) diameter The calculation expressions include: Where, represents the tapering ratio of the multi-core optical fiber coupler; Determine the diameter of the cladding (12) After the size is determined, according to the core arrangement scheme of the target multi-core optical fiber (3), the diameter of the circumscribed circle of the single-clad optical fiber (1) under the tight arrangement is The size is set to the optimal inner diameter of the sleeve (2); According to the cladding diameter of the target multi-core optical fiber (3) Calculate the minimum outer diameter of the casing (2) , the minimum outer diameter The expressions include: ； The step of determining the taper ratio includes: Adjusting the parameters of the single-clad optical fiber (1) according to the mode field diameter A of a single core in the target multi-core optical fiber (3) so that the difference between the mode field diameter B and the mode field diameter A of the single-clad optical fiber (1) in the end of the sleeve (2) that has not been tapered is not greater than a preset threshold; Let the tapering ratio of the multi-core fiber coupler be The initial value is 1, and the taper ratio is When the variable is set as a decreasing variable, the mode field diameter C of the single-clad optical fiber (1) in the tapered end of the sleeve (2) is calculated; When the cone ratio When the value is not 1 and the mode field diameter C is equal to the mode field diameter B, the calculation is stopped and the taper ratio at this time is set to The size of , get the taper ratio The optimal value range of .

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